The pursuit of energy sources that do not require the use of a carbon based fuel, particularly a hydrocarbon, is vigorously pursued. Solar cells are an important technology towards such ends. Solar energy is abundant as the earth receives the equivalent energy from the sun in about an hour as is generated by man in a year. The cost to implementing solar energy involves many factors, but a predominate factor is the efficiency of a solar cell to convert as much of the solar energy reaching the surface of the solar cell to electrical energy as possible. Although many types of solar cells exist, generally differentiated by the nature of the photoactive material used to generate free electrical charge carriers in the cell, the performance of a solar cell of any given photoactive material can vary by a significant amount depending on various designed factors.
Solar concentrators are one way by which performance of a photovoltaic device can be enhanced. In this manner, the light over a given area is focused and directed to a smaller area cell such that more energy than that possible without the focusing can be absorbed by the cell. Solar concentrators are not conducive for use with large-area solar cells as the concentrator would simply divert light directed from one portion of the solar cell to another.
Performance improvements can be achieved by enhancing the efficiency of any given type of solar cell by reducing the optical loss because of reflection from the exposed surface or due to non-absorbance of the light in the solar cell. Anti-reflection coatings enhance solar cell performance at different angles of light incidence. The anti-reflection coating is chosen to have a thickness where the wavelength in the coating material is one quarter the wavelength of the incoming wave. The anti-reflection coating minimizes reflection when its refractive index is the geometric mean of the materials on either side of the coating. Reflectivity can be reduced over a range of wavelengths by including a plurality of anti-reflection layers.
Any roughening of the exposed surface reduces reflection by increasing the probability that reflected light is also projected onto a portion of the surface. Single crystalline silicon wafers can be textured by etching anisotripically along the faces of its crystal planes to leave random sized extended pyramids or even regular inverted pyramids at the silicon surface. Multicrystalline wafers can be textured by photolithography or mechanically using saws or lasers to cut the surface into an appropriate shape.
In contrast, there are limited options to improve light trapping in thin-film solar cells. Many thin-film solar cell technologies have been developed, including devices based on inorganic semiconductors such as amorphous silicon, nano-, micro-, or poly-crystalline silicon, CdTe, and Cu(InxGa1-x)Se2. With a thickness of a few microns or less, thin-film solar cells do not support traditional light-trapping techniques, such as the surface texturing of above. Subwavelength texturing required for thin silicon layers, in addition to increasing the surface area, increases the amount of electrically active centers or defects at the surface. As a result, surface-recombination losses at the transparent conducting oxide/silicon interface increases and the performance of the solar cells decreases. Thus, a novel and relatively simple method is required to enhance light trapping with minor modification and/or addition to the processing steps is desirable.
Another type of emerging thin-film solar cell technology is based on organic semiconductors, including small molecular weight organic compounds (or small molecules) and conjugated polymers. These materials can be easily processed from vacuum (for small molecules) and from solutions (for polymers). Conjugated polymers can also be combined with colloidal inorganic nanoparticles to form hybrid organic-inorganic solar cells that retain the solution processability of polymers. The reflection losses of the incident light in these organic and hybrid solar cells are generally less than in those inorganic semiconductor-based thin film solar cells. This is because the index of refraction for these organic materials, and the typical substrates (glass or plastics) for these organic and hybrid solar cells, is generally much lower than that of the inorganic semiconductors. However, organic semiconductors also possess significantly lower charge carrier mobility, typically 1 cm2/V·s or less, compared, for example, to about 1400 cm2/V·s for electrons in crystalline silicon. Therefore, there exists a trade-off between light absorption and charge collection, as thick films are needed to absorb the incident photons as much as possible, but thin films are more advantageous for complete collection of the photogenerated charge carriers. Hence a means to improve the light absorption efficiency in thin films, while possibly reducing reflection loss, is desired for increasing the overall solar energy conversion efficiency.